Digital versatile discs (DVDs) are information storage devices used for storing prerecorded audio information, movies and computer software. The storage and playback mechanism used in DVDs closely resembles the mechanism used in compact discs (CDs) and DVD players and software use the same laser technology as CD players. Briefly, both DVDs and CDs store information as a pattern of pits formed in a metallic substrate. The pit patterns form digital words and can be read by shining a laser beam on the disc surface and detecting the reflected beam. However, the information storage capacity of a typical DVD is much higher than a CD. Presently available DVDs have a variety of capacities which depend on the technology used to manufacture the discs. Single-layer technologies can be either single or double-sided with capacities of 4.7 gigabytes and 9.4 gigabytes, respectively. Dual layer technologies are available which use single or double sided capacities that hold approximately 8.5 gigabytes per side. This high information storage capacity makes DVDs suitable for storing not only audio information, but also video information and large amounts of computer data as well.
DVD players have many CD player features, such as the ability to play selections in any order desired and the ability to read information from any point on the disc. However, DVDs can store information in several formats. For example, DVDs which are used to store video information, hereinafter called DVD-VIDEO discs, may use various known information compression algorithms, such as MPEG-2 for video compression/decompression. A DVD may also include high fidelity sound as well. In addition, a DVD may also store uncompressed linear pulse code modulated data streams which have sample rates between 48-90 kHertz and are sampled at 16 or 24 bits. Still other DVD versions, hereinafter called DVD-ROM discs, can store digital data for computer use, and the data may also be compressed on these discs.
Although DVD-ROM and DVD-VIDEO discs may share compression algorithms, the data format on DVD-VIDEO discs is significantly different than the data format found on DVD-ROM discs. One important difference is that the data content on DVD-ROM is platform-specific, while DVD-VIDEO discs operate with a platform independent navigation engine for playing interactive movies. This navigation engine requires that the files on the DVD_VIDEO disc be referenced in a predetermined directory structure.
In particular, each DVD-VIDEO disc contains a main directory denoted as a VIDEO_TS directory which contains two types of files distinguished with the file extensions .IFO and .VOB. During playback, these files are sorted by a DVD video player to form video "title" sets, which are groupings of all files necessary to play a particular DVD video "title", for example, a movie. Each video title set is composed of one .IFO file and one or more .VOB files.
A file with the VOB extension contains the actual multimedia data and is called a video object set (VOBS). The location and format of the multimedia data stored in the video object set is defined by the associated .IFO file. In particular, IFO files contain navigational data structures and a processor-independent interpreted language which specifies how the data structures are arranged.
The data structures themselves are composed of various objects called "program chain objects", "program objects", and "cell objects". Program chain objects link related program objects, or particular scenes, within a title and their data structures govern the playback of the program objects. For example, a simple title may contain only one program chain. However, complex titles may contain two or more program chains to allow random access to a variety of programs. The multiple program chain title can play programs linearly, randomly or in a "shuffle" mode.
Each program object in a program chain is composed of elements called "cell objects". These objects instruct a DVD player which portion of the multimedia data in one of the VOB files to decode. In particular, the data structures in a cell object are defined in the IFO file and the multimedia content is found in one of the VOB files. Each cell object directs the DVD player to begin playback at a specific location in the VOB file which is referred to as a video object unit or "VOBU". A VOBU is a container object that includes both multimedia data and playback program commands in an interleaved format.
Input can also be obtained directly from a user by means of controls, such as buttons, which can be displayed under playback program control onscreen along with the multimedia data. The playback program controls both the time duration that a control appears on the screen and the manner that the system responds to the selection of a control by a user. For example, user selection of a button may cause the playback program to jump to a new location on the disk and begin playback at the new location.
The specific program commands which are recognized by a DVD player are controlled by a device independent language and a set of DVD player parameters which define the current state of the DVD player. These commands can be broken into several categories including the following: Set, SetSystem, GoTo, Link, Jump and Compare.
Set commands permit primitive operations, such as compare or assignment operations, to manipulate the values of selected stored parameters. SetSystem commands are used to set the internal system parameters of the player. GoTo commands are used to skip to a specific instruction number in the instruction stream and Link and Jump commands cause program execution to jump to various locations within a title or menu on the disc. Finally, Compare commands allow value testing on either a system or user parameter.
A DVD player system includes a processor and associated memory, which may be part of the player or a separate computer, and display device. The player system is controlled by a playback program running under an operating system. Preferrably, the operating system is a multithreaded operating system such as WINDOWS NT.RTM. or OS/2.RTM.. In such a system, the playback program would typically comprise a system thread which controls many of the player functions, including decompressing and decoding the multimedia and program information on the DVD.
Among the DVD player parameters are several parameters that can be used as registers or counters. When a parameter is used as a register (in register mode), it simply stores data. When a parameter is used as a counter (in counter mode) it can be incremented and decremented under program control. For example, DVD player parameters include a single "system" counter parameter (also called a "navigation timers") and may include up to sixteen "user" counter parameters which facilitate system timing functions that control the time duration of various operations. For example, a user selectable menu might be displayed on a display device for a preselected amount of time.
Conventionally, each of the counter parameters are controlled by a single thread, such as the system thread, which increments, or decrements the counter parameters and resets the counters, as directed by program information in the data stream read from the disc. The system counter parameter is decremented at predetermined time intervals, e.g., every second, and includes logic for performing very specific functions that are used by the player to control system processes. These functions are defined in detail in a specification entitled "DVD 1.0 Specification", the disclosure of which is incorporated herein, in its entirety, by reference. As the aforementioned functions do not form part of the present invention, they will not be described further.
In contrast, the user counter parameters include no logic for performing any system functions and thus, merely maintain a count which is incremented when the system counter parameter is decremented. The data stream read from the disc includes references to selected user counter parameters, thereby utilizing the counter parameters in connection with functions defined in the program commands in the disc data stream.
As noted above, in conventional DVD players, the system thread is responsible for incrementing and decrementing each of the counter parameters. This requires that the system thread repeatedly pause execution of other system functions so that it can poll a system clock to determine if a predetermined time interval has elapsed. The polling may occur many times within the time interval, thus slowing down other system processes. In addition, certain system processes may have priority over the timing functions and may preempt the polling process, delaying a determination that the time interval has elapsed and thereby causing inaccurate system timing and other system errors.
Accordingly, it would be desirable to have a DVD system that ensures efficient utilization of the system thread while also maintaining the accuracy of the counter parameters in the DVD system.